Multiple Entry Tourist Visa.

I have on several occasions been issued a double entry L type for 30 days stay and valid for 6 months. I now see on your web sight that i might be able to apply for a multiple entry 6 months, 1 year or 2 year L type valid for 180 days stay is that correct. I am a uk Citizen residing in the UK. What is the maximum i can apply for with proof of several previous double entries on my passport.